1. Field of the Invention
The invention relates to a field effect transistor (FET) based sensor. More particularly, the invention relates to a FET based sensor with high performance sensitivity and response time.
2. Description of the Prior Art
The ion-sensitive field effect transistor (ISFET) is an electrochemical sensing component disclosed by Piet Bergveld in 1970. The ISFET realizes the combination of minimization and automatic measuring. An ISFET is similar to a metal oxide semiconductor field effect transistor (MOSFET), but the ISFET does not have a conductive gate terminal. Instead, an ion-sensitive membrane is placed over the gate or channel region of the ISFET and is exposed to a sample solution. The wiring of the ISFET is not attached to the gate terminal like a MOSFET, but the wiring of the ISFET is attached to a reference electrode. The reference electrode is separated from the ion-sensitive membrane by the solution. The ion-sensitive membrane modulates the gate charge, and thus the potential difference between the gate and the reference electrode, as a function of the ion concentration in the sample solution. One or more operating characteristics of the ISFET are then measured and used to calculate the ion concentration. Compared to the metal-oxide-semiconductor field effect transistor (MOSFET) utilized in common integrated circuits, the main difference between the ISFET and the MOSFET is that the ISFET utilizes the ion-detecting layer and the electrolyte to substitute the gate terminal of the MOSFET. With the ion selecting function and the characteristic of FET, the ISFET is a new sensor combining electrochemistry and semiconductor.
The use of ISFET for sensing ion is known. For example, the U.S. Pat. No. 5,833,824 discloses such a sensor. One application of the ISFET sensors is in the process control of the food and beverage. It is because that the traditional pH glass sensor is unsuitable and prohibited from the food and beverage.
It is also known that different materials have different sensing characteristics when used as ion-sensing membranes of pH ISFETs. In 1970 Bergveld of the Technical University Twente (TH Twente) described the principle of the ISFET. The ISFET includes semi-conductor material, for example p-type silicon, which is provided with an oxide surface, such as silicon dioxide (SiO2), silicon nitride (Si3N4), alumina (Al2O3), zirconia (ZrO2) and tantalum oxide (Ta2O5). However, the strength of the ion-sensing membrane of the pH ISFET is usually not enough. The ion-sensing membrane of the pH ISFET is going to be etched under some detection processes, for instance, the Cleaned In Place (CIP) process with a 2% NaOH solution at 85° C.
Besides the pH ISFETs, there are several types of ISFET structures. For example, a high-electron-mobility transistor (HEMT) can serve as an ISFET. The U.S. patent Pub. No. 2008/0203431A1 discloses such a sensor. Please refer to FIG. 1. FIG. 1 is a sectional view illustrating an ISFET 1 in prior art. The ISFET 1 in FIG. 1 is an AlGaN/GaN high-electron-mobility transistor. As shown in FIG. 1, the ISFET 1 includes a substrate 10, an AlN layer 11, GaN layers (12, 14), an AlGaN layer 13, a source terminal 15 and a drain terminal 16. The AlN layer 11 serves as a nucleation layer. The GaN layer 12 serves as a buffer layer. Therein, a two-dimensional electron gas (2DEG) 17 is formed at the interface between the GaN layer 12 and the AlGaN layer 12, and more particularly, located on side by the GaN layer 12. As shown in FIG. 1, the exposed gate area 140 can detect the attached ions, such that the surface potential of the ISFET 1 is modified to affect the density of the 2DEGs 17, so as to change the source-to-drain current flow.
By aforesaid interaction, the AlGaN/GaN HEMT can server as an ISFET. However, the sensitivity and the response time of the AlGaN/GaN HEMT-type ISFET is not ideal. It is not sensitive and fast enough to achieve the real-time detection and high sensitivity needed in modern sensor technologies.
Therefore, the invention discloses an ISFET sensor with high sensitivity and short response time, so as to solve said problems.